Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat color panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology. On one hand, the plasma display technology gives the possibility of nearly unlimited screen size, also of attractive thickness, but on the other hand, it generates new kinds of artefacts which could reduce the picture quality. Most of these artefacts are different from the known artefacts occurring on classical CRT color picture tubes. This different appearance of the artefacts makes them more visible to the viewer since the viewer is used to see the well-known old TV artefacts.
The invention deals with a specific new artefact, which is called “dynamic false contour effect” since it corresponds to disturbances of gray levels and colors in the form of an apparition of colored edges in the picture when an observation point on the matrix screen moves. This kind of artefact is enhanced when the image has a smooth gradation like when the skin of a person is being displayed (e.g. displaying of a face or an arm, etc.). In addition, the same problem occurs on static images when observers are shaking their heads and that leads to the conclusion that such a failure depends on the human visual perception and happens on the retina of the eye.
Several approaches have been proposed to reduce or eliminate false contour effect. As, the false contour effect is directly related to sub-field organization of the used plasma technology, most of the approaches try to optimize the sub-fields organization of the plasma display panels. The sub-field organization will be explained in greater detail below, but for the moment, it should be noted that it is a kind of decomposition of the 8-bit gray level in 8 or more lighting sub-periods.
One approach proposed by Pioneer consists in using continuous sub-fields arrangements with no holes in the code. However, due to the reduction of the number of gray levels to the number of sub-fields, the picture quality is really damaged.
Another approach for compensating the dynamic false contour effect has been patented by Deutsche Thomson-Brandt GmbH in EP patent application No. 01 250 158.1. The patented method is based on gravity centered coding using only sub-field sequences having a similar progression of their lighting center of gravity as the progression of the video levels themselves. This is achieved through a reduction of the available video levels which will be recovered through dithering. Such a coding, while very good for moving sequences, is less efficient in case of still or non-moving pictures because of the dithering noise.